Aromatic diisocyanates which contain sulphonic acid groups are known (German Offenlegungsschrift No. 1,939,911). The reaction of tolylene diisocyanate with sulphur trioxide, for example, results in a crystalline, high melting product which dissolves by a rapid reaction in sodium hydroxide solution. The resultant product may be used, for example, for the preparation of anionic polyurethane dispersions. From German Offenlegungsschrift No. 2,227,111 which corresponds to U.S. continuation-in-part application Ser. No. 528,319 filed on Dec. 17, 1974 it is also known that polyisocyanates which contain sulphonic acid and/or sulphonate groups, and in which uretdione derivatives are present in addition to other reaction products, may be obtained by the sulphonation of liquid multicomponent mixtures of aromatic polyisocyanates (see also German Offenlegungsschrift No. 2,359,614 which corresponds to U.S. patent application Ser. No. 527,473 filed on Nov. 26, 1974).